XML is a mark up language which allows you to define your own tags, and to define data rather than format it. The overall concepts of XML are quite well understood but there is little information available about issues of developing applications using XML. By considering the decisions to be made at various stages of the project, it will help readers understand the various aspects of XML and its related technologies. It will stress the benefits of XML in informational systems, where it can be used to separate content from presentation, and in transactional systems, where it can act as a low-cost alternative to EDI protocols.